The present invention relates to an imaging method which is effective in rapidly imaging low flow rate fluid in a human body by a dislocation imaging device which utilizes nuclear magnetic resonance phenomenon (hereinafter referred to as MRI).
The following articles 1) to 9) disclose the technologies relating to the present invention.
1) IEEE Trans. on Medical Imaging, MI-5, No. 3, pp. 140-151, 1986. PA0 2) Magnetic Resonance in Medicine 6, 274-234, 1988. PA0 3) Magnetic Resonance in Medicine 7, 35-42, 1988. PA0 4) Journal of Magnetic Resonance 62, 12-18, 1985. PA0 5) Magnetic Resonance in Medicine 3, 140-145, 1986. PA0 6) Magnetic Resonance in Medicine 4, 9-23, 1987. PA0 7) Magnetic Resonance in Medicine 10, 324-337, 1989. PA0 8) U.S. Pat. No. 4,788,500. PA0 9) Magnetic Resonance in Medicine 14, 222-229, 1990.
A prior art method for imaging fluid in the NMR imaging is discussed in detail in the reference 1). A principle of selection of blood flow utilizes a gradient magnetic field pulse which causes a change in a phase by flow or movement, which is called a flow encode pulse. Where the flow encode pulse is present along the direction of flow of the blood flow, a change of phase is produced in an excited spin in the blood flow in accordance with a flow rate. By subtracting between two images reconfigured based on NMR signals (hereinafter referred to as signals) detected in a dephase sequence which includes the flow encode pulse and a rephase sequence which does not include the flow encode pulse, image data of only the blood flow can be detected. A principle thereof is as follows. The blood flow in a blood vessel is a laminar flow which is of high flow rate at a center and of low flow rate at a periphery. Accordingly, when it is imaged by the dephase sequence, the excited spin has different phase change depending on a distance from the center of the blood vessel. As a result, signal data projected on a plane parallel to the blood flow has random orientation of the phases of the spins and resultant vectors cancel each other so that no signal is produced from the blood vessel or a signal amplitude is very small. On the other hand, when it is imaged by the rephase sequence, the phases of the spins which have once disturbed are reordered as the blood flows and if the signal is measured at a specific timing, the signal without phase change can be detected. Accordingly, when it is imaged by the rephase process, a signal may be detected even from the blood flow which includes a laminar flow. A signal may be detected from a static area by either one of the sequences, but the signals of the static area are cancelled out by subtracting the images produced by the two sequences so that the signal of only the blood vessel is produced. This method is generally called a subtraction method.
A method for imaging a signal produced in a steady-state free precession is discussed in the references 2) to 4). When an RF pusle is irradiated at an extremely short repetition cycle compared to a relaxation time of the excited spin, the steady state free precession occurs so that the NMR signal is periodically produced in the imaging area in a stable manner, and a free induction decay (FID) signal is produced immediately after the RF pulse and a time-reversed free induction decay signal is produced immediately before the next RF pulse. It is known that the time-reverse FID signal has a similar property to that of an echo signal produced by the RF pulse of the two-preceding stage and the RF pulse of the preceding stage, that is, an echo signal having an echo time (TE) which is double of TR.
For the time-reversed FID signal, the two sequentially applied RF pulses function as a 90.degree. pulse and a 180.degree. pulse in the spin echo pulse sequence. As a result, the RF pulse in the preceding stage functions to invert the phase of the NMR signal produced by the RF pulse of the preceding stage thereof. In this case, a gradient magnetic field is applied such that the phase-inverted signal converges, then diverses and is reconverged.
A method for drawing slow flow rate fluid by utilizing the signal produced in the SSFP state is discussed in the references 5) to 8). Particularly in the reference 8), the method is discussed in detail. It describes that when the time-reversed FID signal produced in the SSFP state is to be detected, the interval of application of the RF pulses, that is, the repetition time is changed or the magnitude of the gradient magnetic field applied to detect the signal is changed to rotate the phase of the excited spin of the fluid so that two images are formed in the two dephase and rephase sequences having different gradient magnetic field magnitudes and the slow flow rate fluid is drawn by the subtraction between those images.
In this case, depending on the degree of the phase change of the spin, a large signal of the fluid may not be produced by the subtraction. This is explained with reference to FIGS. 1 and 2.
FIG. 1 shows a flow rate distribution of the blood flow. Numeral 1 denotes blood and numeral 2 denotes a direction of flow. The flow rate is high at points a and d, and low at points b and c.
FIG. 2 show the phase rotations at the points a to d of FIG. 1 and combined signals of the signals at the points a to d in two different phase sequences. Mo represents a magnitude of signal in the SSFP state.
As shown in FIG. 5, when the phases of the resultant signals are in the substantially same direction, a differential signal is very small. In the example shown in FIG. 2, EQU .vertline.Sd.sub.1 -Sd.sub.2 .vertline..apprxeq.2.8 Mo
Depending on the phase angles of Sd.sub.1 and Sd.sub.2, the signal is smaller.
In the reference 8), no consideration is paid to a method for drawing slow flow rate fluid based on data derived from measurement in sole dephase sequence or rephase sequence.
A maximum intensity projection method and a minimum intensity projection method in the image processing are discussed in detail in the reference 9). In the maximum intensity projection method, when a two-dimension image projected in any direction is to be produced based on three-dimensionally measured data and data which is three dimensionally measured by two-dimension multi-slice, a projection image is formed by using a maximum intensity pixel of a plurality of pixels of the three-dimension source data as a target pixel. In the minimum intensity projection method, a minimum intensity pixel is used as a target pixel.